The present invention relates to a control system for a heat pump which causes the heat pump to go into its defrost mode when a particular inside heat exchanger condition is reached.
The present invention also relates to a heating system employing both a heat pump and a source of supplemental heat, such as a resistance heat furnace or a fossil fuel furnace, and relates in particular to a control system whereby greater efficiency in the utilization of the two heat sources is realized.
A heat pump utilizes a compressor and a refrigerant recirculation system including a condenser and evaporator to provide both cooling in the warmer seasons of the year and heating in the winter. When heating, the high temperature condenser is located in the interior space and the lower temperature evaporator is located outdoors to extract heat from the outside air and transfer it to the interior space through the condenser.
Since heat pumps utilize outside ambient air in contact with the evaporator as the heat source during the winter months, they operate efficiently only when the outside air temperature is above a certain level, such as 40.degree., for example. In regions of colder average winter temperatures, supplemental heat, such as is supplied by fossil fuel furnaces or resistive heat, is necessary in order to maintain the temperature within the building at the desired level. As the outside temperature drops, there is less heat available for transfer to the interior condenser, so that the system eventually reaches a point where the heat transfer is not adequate to satisfy the heat demand called for by the thermostat Furthermore, as the outside ambient temperature drops, the efficiency of the heat pump suffers because of frost buildup on the evaporator coils, which occurs at a greater rate with a progressive decrease in the outside ambient temperature.
In the defrost cycle, the heat pump is run in the reverse direction to transfer heat from the warmer indoor condenser coil to the outside evaporator coil, thereby melting the frost. Following the defrost cycle, normal operation can be resumed, assuming that heat is called for by the thermostat. Of course, during the defrost cycle of the heat pump, heat is not being supplied to the building, and the supplemental heat must be relied on to maintain the desired ambient temperature. This requires that both the supplemental heat unit and the heat pump be operated simultaneously, the former to maintain the desired heat level within the building, to provide heat for the defrost cycle, and the latter to eliminate the frost build-up so that the heat pump can return to normal operation.
Whenever the outside ambient is below that which permits adequate transfer of heat, both the heat pump and the supplemental heat source are operating simultaneously, with greater energy demand than with the heat pump operating alone or with the supplemental heat operating alone. When the heat pump and supplemental heat source are operating together beyond a certain portion of the heat cycle, there is greater energy consumption than if only the supplemental heat source alone is used for a given quantity of heat delivered. During the defrost cycle of the heat pump, energy is required to heat the outside evaporator coils, and supplemental heat is necessary to maintain the desired inside temperature level called for by the thermostat and to provide adequate heat for defrost operation. Accordingly, if frequent and lengthy defrost cycles are necessary to maintain the evaporator coils free of frost, less energy will be consumed by operating the supplemental heating alone and shutting down the heat pump entirely. This is true even though the heat pump operation is generally more efficient than supplemental heating, for example resistance or fossil fuel burning, depending on the outside temperature and humidity conditions. The buildup of frost on the evaporator coils is a function of the outside ambient temperature and also the dew point. If the dew point is high, moisture will condense on the evaporator coils and turn to frost at a higher temperature than if the dew point is lower.
Prior art control of heat pump operation is generally accomplished by means of an electromechanical thermostat mechanism, with separate temperature sensors for each state of heat pump system operation. Furthermore, there are defrost timers, relays, and pressure and temperature sensors utilized to control system defrost cycling. The weather and condition of the heat pump system form a complex set of factors that are constantly changing, thereby making it very complex to determine the combination of heat pump and resistive heating which renders maximum efficiency. To maintain the heat pump system in the most efficient state would require the user to continually measure all of these factors and perform complex computations. Accordingly, prior art heat pump installations do not have the means available to the user to efficiently operate the heat pump systems in the lowest energy demand state while maintaining the temperature of the building at the desired level.
To summarize, the current problem with heat pump installations is that their advantage over other methods of heating exists only when the heat pump operates without supplemental heat. The more frequently that the heat pump operates with supplemental heat, either during its heating cycle when the outside ambient temperature is so low that the heat pump is not able to satisfy the heat demand, or during its defrost cycle, the less advantage there is in terms of energy efficiency over other heating plants, such as resistance or fossil fuel furnaces. This has resulted in heat pumps being used more often in regions where the outdoor temperatures are sufficiently high during the winter months that the need for supplemental heat is infrequent, such as in the Southern and Southwestern regions of North America. The use of heat pumps in cooler Northern climates, particularly in those climates where the air humidity is high during the winter months, requires very complex controls which, although perhaps they can be justified for large buildings, are not feasible for domestic and smaller commercial and industrial installations.
All heat pumps have a built in control system for switching the heat pump over to its defrost mode. In the defrost mode, a reversing valve connected at the discharge of the compressor reverses the flow of refrigerant through the condenser and the evaporator. In the winter, the indoor heat exchanger coil functions as the condenser and receives compressed refrigerant which then gives up heat to interior ambient air blown over the indoor heat exchanger coil by a fan. The outdoor coil functions as the evaporator and absorbs heat from the outdoor ambient. When defrosting of the outdoor coil is necessary, the heat pump control system activates a relay or the like which reverses the reversing valve, thereby causing refrigerant from the compressor to be pumped through the outdoor heat exchanger coil, which now functions as a condenser, and gives up heat to the exterior surface of the coil so as to melt any ice or frost that has built up thereon. After a given period of time, as determined by a timer in the heat pump defrost control system, the reversing valve is reversed and the indoor coil again functions as the condenser.
A variety of techniques have been utilized to determine when the heat pump should go into its defrost mode, such as monitoring the outdoor coil temperature, monitoring the outdoor ambient temperature and monitoring compressor motor current. In a simpler system, a timer causes the heat pump to go into the defrost mode on a periodic basis. In some systems, a variety of detected conditions are analyzed by a microcomputer to determine if and when the heat pump should defrost.
A disadvantage to all of the prior art techniques for determining when the heat pump should defrost is that they rely on conditions which are either unrelated or, at best, only indirectly related to the efficiency of the heat pump. For example, monitoring of outdoor temperature may provide some measure of how often defrosting is necessary, but systems utilizing outdoor ambient temperature as the monitored condition for defrost will often go into the defrost mode when there is little, if any, build up of frost on the outdoor coil. It should be noted that the defrost mode for a heat pump is extremely inefficient in terms of heating a building because it causes the internal coil to become the colder, evaporator coil over which the interior air is passed during the defrost cycle. Accordingly, not only is the heat pump not operating, but it is operating in a manner which is counterproductive to the heating of the building. On the other hand, in some prior art systems, defrost cycling may not be initiated even though it is needed in that the system is operating at very low efficiency. For example, if the outdoor humidity is high but the system only monitors outdoor temperature to determine defrost frequency, frost may form on the outdoor coil and cause the system to run at very low efficiency until defrost is initiated by the system timer.